1. Field of the Invention
The present invention relates to a crystalline polymer microporous membrane, a method for producing the crystalline polymer microporous membrane, and to a filtration filter using such crystalline polymer microporous membrane.
2. Description of the Related Art
Microporous membranes have long since been known and widely used for filtration filters, etc. As such microporous membranes, there are, for example, a microporous membrane using cellulose ester as a material thereof (see U.S. Pat. No. 1,421,341), a microporous membrane using aliphatic polyamide as a material thereof (see U.S. Pat. No. 2,783,894), a microporous membrane using polyfluorocarbon as a material thereof (see U.S. Pat. No. 4,196,070), a microporous membrane using polypropylene as a material thereof (see West German Patent No. 3,003,400), and the like.
These microporous membranes are used for filtration and sterilization of washing water for use in the electronics industries, chemical for semiconductor production, water for medical use, water for pharmaceutical production processes and water for use in the food industry. In recent years, the applications of and amount for using microporous membranes have increased, and microporous membranes have attracted great attention because of their high reliability in trapping particles. Among these, microporous membranes made of crystalline polymers are superior in chemical resistance, and in particular, microporous membranes produced by using polytetrafluoroethylene (PTEF) as a raw material are superior in both heat resistance and chemical resistance. Therefore, demands for such microporous membranes have been rapidly growing.
Generally speaking, microporous membranes have a low filtration flow rate (i.e., a short lifetime) per unit area. In the case where the microporous membranes are used for industrial purposes, it is necessary to align many filtering units to increase the membrane areas. For this reason, a reduction in the cost for the filtering process is appreciated, and thus an extension of the filtering lifetime is desired. To this end, there are various proposals for a microporous membrane effective for preventing or slowing down reductions in flow rate due to clogging, such as an asymmetric membrane in which pore diameters are gradually reduced from the inlet side to the outlet side.
Moreover, another proposal is a microporous membrane of a crystalline polymer, which has a larger average pore diameter on a surface of the film than that on the back surface thereof, and has the pores whose average diameter continuously changes from the surface to the back surface (see Japanese Patent Application Laid-Open (JP-A) Nos. 2007-332342). According to this proposal, fine particles are efficiently captured by the filter and the lifetime of the filter is improved, by performing filtration using, as the inlet side, the plane (i.e. the surface) having the larger average pore diameter.
A hydrophilization method of a crystalline polymer microporous membrane having an asymmetric pore structure proposed in JP-A No. 2009-119412 is that an exposed surface of a crystalline polymer microporous membrane having an asymmetric pore structure is subjected to a hydrophilization treatment, such as by immersion in aqueous solution of hydrogen peroxide or water-soluble solvent, laser irradiation, or chemical etching.
However, in the case of the crystalline polymer microporous membrane having an asymmetric pore structure, the membrane has a heated surface, unheated surface, and an inner part in between these surfaces, and the polymerization degree of the crystalline polymer varies in each area. The aforementioned hydrophilization methods cannot uniformly hydrophilize the entire membrane of such structure, and a hydrophilization process needs to be performed on each area separately depending on the polymerization degree thereof so as to uniformly hydrophilize the entire membrane, and the requirement for these separate hydrophilization processes results in low efficiency. Moreover, the membranes hydrophilized by these methods do not have sufficient hydrophilicity, and the filtration flow rate and lifetime thereof are also insufficient. In addition, the hydrophilization method by applying an ultraviolet laser beam and ArF laser beam has a problem such that the radiation of the ultraviolet laser beam and ArF laser beam may damage the membrane, and thus the strength of the membrane may be decreased.
As a hydrophilization method of a crystalline polymer microporous membrane a method disclosed in JP-A No. 2007.503862 proposes modification at least part of an exposed surface of the porous membrane which contains a cationic polymerizable monomer and a cationic polymerization initiator by polymerizing the cationic polymerizable monomer. Moreover, it proposes that the crystalline polymer microporous membrane may further contain a functional monomer containing quaternary ammonium salt, etc.
However, in this proposal the crystalline polymer microporous membrane has poor alkali resistance, and filtration rate, and life time of the porous membrane cannot be improved to a satisfiable degree.
Moreover, recently in semiconductor production, in order to obtain ultra-ultrapure water having outstandingly high purity for use in the semiconductor production, a filter capable of simultaneously capturing fine particles and metal ions existing in a concentration of less than ppm is desired. Particularly, desired is a precision filtration filter having capturing ability of metal ions, and high resistance to chemicals such as acid, alkali, and an oxidizing agent, and causing less eluted material. For such purpose, conventionally, it has been attempted to give ion exchange function and/or ion adsorption ability to a microporous membrane itself. As the crystalline polymer microporous membrane having the ion adsorption ability, proposed in JP-A No. 09-511948 is a porous membrane containing a functional group reactive with a polar group activated in the membrane, and provided with any ligand having affinity to a specific ion.
However, the bond between the ligand and the membrane is poor in alkali resistance, such as amide bonding, ester bonding. There is a problem of difficulty in use of the membrane in a semiconductor washing step, or the like. There is also problem that numberless steps are necessary for bonding the ligand, and that the filtration flow rate, and life time of the porous membrane cannot be improved to a satisfiable degree.
As the crystalline polymer microporous membrane having the ion adsorption ability, proposed is a porous membrane on which surface a functional group for capturing is introduced, wherein at least part of epoxy groups is substituted with a chelate-forming group or an ion-exchange group (see JP-A No. 2007-313391).
However, in this proposal, it is essential to use a compound substantially having an ester bonding, and there is a problem that alkali resistance is poor, and filtration flow rate, and life time of the porous membrane cannot be improved to a satisfiable degree.
Moreover, a method for modifying a surface of polyethylene terephthalate or polyethylene naphthalate by fixing a fluorine compound and a crosslinking agent having an isocyanate group on the polyethylene terephthalate or polyethylene naphthalate is proposed (JP-A No. 2008-37912).
However, in this proposal, since the functional group bonding with the fluorine compound does not exist on a surface of a polymer, metal ions cannot be captured.
There is a proposal of a method for improving wettability and chemical resistance of a fluorine resin molded product by covering a surface of the fluorine resin molded product with an alcohol solution containing an ultraviolet absorbing compound and a fluorosurfactant (JP-A No. 07-207049).
However, since this proposed fluorine resin molded product is not a porous membrane of a crystalline polymer, metal ions cannot be captured. There is also a problem that water resistance, acid resistance and alkali resistance are insufficient, since the ultraviolet absorbing compound is not crosslinked onto the fluorine resin molded product.
Accordingly, there is currently a demand for a crystalline polymer microporous membrane having high water resistance, high acid resistance, high alkali resistance, high ion adsorption ability, high hydrophilicity, long lifetime as a filter, and excellent filtration flow rate, a method for producing a crystalline polymer microporous membrane, which can efficiently produce the aforementioned crystalline polymer microporous membrane, and to a filtration filter using the aforementioned crystalline polymer microporous membrane.